1. Field of the Invention
The present invention relates to an X-ray computerized tomography apparatus which reconstructs an image of a plane section of a human body under examination from X-ray signals (projection data) obtained by detecting the variation in X-ray transmission through the plane section of the body at various angles.
2. Description of the Related Art
The important problems that confronted X-ray computerized tomography apparatus since its development were the improvement in image quality and the reduction of scan time required. The helical scan, which is an epoch-making solution for these problems, is attained by combining continuous rotation of a gantry, to which an X-ray tube is mounted, around a subject under examination and continuous movement of the subject parallel to the axis of rotation of the gantry. With this helical scan, assuming the subject to be fixed, the X-ray tube will move helically along the body axis of the subject.
The helical scan has the following drawbacks. In the helical scan, the angle of the rotating gantry changes periodically (hereinafter referred simply to as the angle) and moreover projection data are acquired in each of successive locations (hereinafter referred to as the Z-axis coordinate) along the slice axis (hereinafter referred to as the Z axis) parallel to the body axis of the subject. In order to reconstruct a single tomogram of a plane section of the subject using such projection data, the simple interpolation method requires two rotations of projection data (data obtained during two rotations) with the Z-axis coordinate of the plane section taken as center. Distance interpolation is performed on two rotations of projection data to obtain one rotation of projection data. The distance interpolation is a process of taking a weighted average of two pieces of data obtained at the same angle of rotation but at different Z-axis coordinates according to their distance from the Z-axis coordinate of the plane section. A tomogram of that plane section of the human body is reconstructed from one rotation of projection data thus obtained.
Thus, in the helical scan, all-angular projection data required to reconstruct a single tomogram will be dispersed within a range of distance through which the subject (laid down on a top board) moves while the gantry makes two rotations, which will result in decreased reliability of tomograms.
Opposed beam interpolation was developed to minimize this difficulty. Data acquired in positions with an angular difference of 180 degrees therebetween principally contain information about the same tissues (X-ray absorption information). Thus, the opposed beam interpolation handles projection data acquired at an angle and at a Z-axis coordinate as projection data (hereinafter referred to as opposed data) which would be obtained at the angle differing by 180 degrees from that angle and at the same Z coordinate. With such opposed beam interpolation, projection data from all angles required to reconstruct a single tomogram will be dispersed within a range of distance through which the subject (the top board) moves while the gantry makes one rotation around the subject. In principle, therefore, the opposed beam interpolation allows the reliability of tomograms to be increased by a factor of two over the simple interpolation.